DC (direct current) or AC (alternate current) servo motors used for an actuator in industrial robots and the like are generally required to have quick response for controlling positions of the robots quickly with high accuracy, and also sufficient durability to bear frequently repeated operation of startup, stoppage, and change of normal and reverse rotation. FIG. 4 exemplifies a DC servo motor including a general construction of the servo motor. The servo motor 20 has a motor casing 21 with left and right side walls each having bearing metal 23 with which rotatably supported are journal portions 22a of a motor rotary shaft (rotary) 22 made of steel. The motor rotary shaft 22 comprises an armature 24, an armature coil 25, a commutator 26 and the like. A permanent magnet 27 is fixed on the casing 21 at a position to face the armature 24, and the rotary shaft 22 is connected at its one end with a position/speed detector 30 and at an output portion 22b of the another end is connected with a speed reducer 29, such as gears. The servo motor 20 constructed with the above structure is required to have high durability at the journal portions 22a thereof in association with the bearing metals 23 in order to meet the aforesaid requirement.
Hence, specific kinds of steel material having high durability are selected for the motor rotary shaft 22 to improve the durability at its journal portions 22a. This, however, leads to problems such as increases in cost of material and weight.
Accordingly, an object of the invention is to improve the durability of the journal portions of the motor rotary shaft without increases in cost of material and weight.